The structure and operation of wireless communication systems is generally known. Examples of such wireless communication systems include cellular systems and wireless local area networks, among others. Equipment that is deployed in these communication systems is typically built to support standardized operations, i.e. operating standards. These operating standards prescribe particular carrier frequencies, modulation types, baud rates, physical layer frame structures, MAC layer operations, link layer operations, etc. By complying with to these operating standards, equipment interoperability is achieved.
In a cellular system, a governmental body licenses a frequency spectrum for a corresponding geographic area (service area) that is used by a licensed system operator to provide wireless service within the service area. Based upon the licensed spectrum and the operating standards employed for the service area, the system operator deploys a plurality of carrier frequencies within the frequency spectrum that support the subscribers' subscriber units within the service area. These carrier frequencies are typically equally spaced across the licensed spectrum. The separation between adjacent carriers is defined by the operating standards and is selected to maximize the capacity supported within the licensed spectrum without excessive interference.
In cellular systems, a plurality of base stations is distributed across the service area. Each base station services wireless communications within a respective cell. Each cell may be further subdivided into a plurality of sectors. In many cellular systems, e.g., GSM cellular systems, each base station supports forward link communications (from the base station to subscriber units) on a first set of carrier frequencies and reverse link communications (from subscriber units to the base station) on a second set of carrier frequencies. The first set and the second set of carrier frequencies supported by the base station are a subset of all of the carriers within the licensed frequency spectrum. In most, if not all cellular systems, carrier frequencies are reused so that interference between base stations using the same carrier frequencies is minimized but so that system capacity is increased. Typically, base stations using the same carrier frequencies are geographically separated so that minimal interference results.
Both base stations and subscriber units include Radio Frequency (RF) transmitters and RF receivers. These devices service the wireless links between the base stations and subscriber units. Each RF receiver typically includes a low noise amplifier (LNA) that receives an RF signal from a coupled antenna, a mixer that receives the output of the LNA, a band-pass filter coupled to the output of the mixer, and a variable gain amplifier coupled to the output of the mixer. These RF receiver components produce an Intermediate Frequency (IF) signal that carries modulated data.
In order to improve the signal-to-noise ratio of an RF signal presented to the mixer by the LNA, the gain of the LNA is adjusted. In adjusting the gain of the LNA, great care must be taken. While maximizing the gain of the LNA serves to increase the Signal to Noise Ratio (SNR) of the RF signal, if the LNA gain is too large, the mixer will be driven into non-linear operation and the IF signal produced by the mixer will be distorted. Such is the case because a non-linear operating region of the mixer resides at an upper boundary of its operating range. The input power level at which output power is compressed by 1 dB for the mixer is often referred to as the 1 dB compression level. Thus, it is desirable to have the LNA provide as great an amplification of the received RF signal as possible prior to presenting the amplified RF signals to the mixer without driving the mixer into non-linear operation. During most operating conditions, the gain of the LNA may be set by viewing the input power present at the LNA and by setting the LNA gain to produce an output that causes the mixer to operate in a linear region.
However, when intermodulation interference exists, this technique for setting the LNA gain does not work. Intermodulation interference in the mixer occurs when the mixer receives two or more strong RF interference signals at certain neighboring channels (in addition to the desired signal) that cause the mixer to produce intermodulation components at the IF, the same frequency as the desired signal. This problem is well known and is a non-linear phenomenon associated with the operation of the LNA or mixer. The intermodulation component at the frequency of the desired signal is a third-order intermodulation component, IM3. In order to minimize intermodulation interference, the gain of the LNA should be reduced. However, reducing the gain of the LNA also reduces the SNR of the signal produced by the mixer. Thus, competing operational goals exist by the competing goals of increasing SNR by increasing the gain of the LNA and by reducing the effects of intermodulation interference by reducing the gain of the LNA.
While this problem is well known, its solution is not. Some prior techniques have simply avoided high LNA gain when wideband received signal strength (across some portion of the operating range of the RF receiver) was approximately equal to the narrowband received signal strength (at the IF). Further, when the wideband received signal strength was significantly greater than the narrowband received signal strength, the gain of the LNA was set to a low level. These operations addressed the issue of the existence of interferers. However, it did not consider whether intermodulation interference existed. Thus, the LNA gain was often times not optimally or acceptably set.
Additional difficulties arise when the receiver operates in a Time Division Multiple Access (TDMA) system in which transmissions from differing locations are non-time-aligned. Prior techniques for setting the gain of the LNA typically consider whether potentially interfering signals, i.e., signals that produce an intermodulation product, exists during a guard period immediately preceding a received data frame. However, in TDMA systems, data frames of differing base stations are typically not time aligned. Thus, in some operations, interfering signals that produce an intermodulation product in the receiver are not present during a guard period that precedes a time slot intended for the receiver but become active during the time slot intended for the receiver. When the interfering signals become active during the time slot, an IM3 component is produced in the mixer, causing the receiver to fail to correctly receive the data contained in the time slot.
Thus, there is a need in the art to improve the operational characteristics of the LNA in order to maximize signal-to-noise ratio and to minimize the effects of intermodulation interference.